unchecked assignment 'java.util.list' to 'java.util.list

Unchecked assignment: ‘java.util.List’ to ‘java.util.Collection’

I am having an adapter where i have two lists one list is for InvestorsList where it comes with the list of investors and the other list is called investorListFull which is used to filter results when searching.

Below is how i have declared the lists

Below is how the lists are assigned in my recyclerview adapter constructor

Below is how i am filtering results in investors list

I am getting Unchecked assignment error in publish results investorList.addAll((List) filterResults.values);

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I am getting Unchecked cast error in publish results investorList.addAll((List) filterResults.values);

That’s because you’re doing an unchecked cast. Actually, you’re doing both a checked and an unchecked cast there.

(List) filterResults.values is a checked cast. This means that the Java compiler will insert a checkcast instruction into the bytecode to ensure that filterResults.values (an Object ) really is an instance of List .

However, investorList.addAll expects a List<Investor> , not a List . List is a raw type . You can pass a raw-typed List to a method expecting a List<Something> ; but this is flagged as unsafe because the compiler can’t guarantee that it really is a List<Something> – there is nothing about the List that makes it a “list of Something “, because of type erasure. The compiler can insert a checkcast instruction to ensure it’s a List ; but not one to ensure it’s a List<Something> : this fact is unchecked .

What it’s saying with the warning is “there may be something wrong here; I just can’t prove it’s wrong”. If you know for sure – by construction – that filterResults.values really is a List<Investor> , casting it to List<Investor> is safe.

You should write the line as:

Note that this will still give you an unchecked cast warning, because it’s still an unchecked cast – you just avoid the use of raw types as well.

If you feel confident in suppressing the warning, declare a variable, so you can suppress the warning specifically on that variable, rather than having to add the suppression to the method or class; and document why it’s safe to suppress there:

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Unchecked assignment: 'java.util.List' to 'java.util.List<ua.lv.hoy.entity.Customer>' #184

ghost commented Feb 8, 2018

The text was updated successfully, but these errors were encountered:

halyafg commented Feb 10, 2018

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ghost commented Feb 26, 2018

Halyafg commented feb 27, 2018.

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fastjson对泛型的反序列化

fastjson未指定泛型具体类型

Fastjson typereference指定泛型具体类型.

fastjson反序列化时使用 TypeReference 指定泛型的具体类型：

这个方法比较优雅，比较保险。

告警本身没什么影响，不影响编译，甚至于告警本身都可以被压制，但是并不能忽略告警：

编译没有问题，但是运行出现类型转换异常：

可以看到，是在迭代器获取具体元素时， String 类型强转为 Integer 导致类型转换异常 ClassCastException ，查看 class 文件的反编译结果，也证实了这一点：

由于没有给fastjson反序列化指定泛型具体类型，fastjson内部是以 JSONObject 存在， JSONObject 无法转为 User ，但是 JSONObject 是可以转为 Object ，毕竟 Object 是所有类的根基类。

运行输出正常， TypeReference 不仅消除反序列化的告警，还告诉fastjson泛型的具体类型。

Java中含有泛型的 JSON 反序列化问题

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Trying to understand an unchecked conversion warning

Using @Mock as a Method Parameter with Mockito

As with many other Java developers, I heavily utilise Mockito as a mocking framework for unit testing.

I'll often end up with tests that look like this:

In this case, I want to create a List and check it's returned by the caching layer I'm testing.

However, this is a good example of where inlining our mock calls is bad, because IntelliJ warns us:

This is quite annoying, and pollutes our codebase with warnings that we really want to avoid.

One solution would be to make it a field of the test class, and @Mock it there, but in this case it's a throwaway mock that is only required in this test, not in all tests (therefore wouldn't make as much sense to make a field variable).

Fortunately, we can utilise something my colleague Lewis taught me the other week, and use @Mock as a method parameter when using JUnit5.

This gives us the following test:

This would of course, make our tests more readable if we had many mocks being set up!

Note that you also need to make sure you're using Mockito's mock setup:

This has been tested as of spring-boot-starter-test v2.2.4.RELEASE, and works in Mockito 2+ with JUnit5+.

I have raised a PR to add it to the Mockito 3.x documentation , but not 2.x as per request of the Mockito core team .

This post's permalink is https://www.jvt.me/posts/2020/06/27/mockito-mock-parameter/ and has the following summary:

Using `@Mock` on method parameters to reduce manual mock setups with Mockito.

The canonical URL for this post is https://www.jvt.me/posts/2020/06/27/mockito-mock-parameter/ .

Content for this article is shared under the terms of the Creative Commons Attribution Non Commercial Share Alike 4.0 International , and code is shared under the Apache License 2.0 .

# blogumentation # java # mockito .

This post was filed under articles .

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Java warning "unchecked conversion"

Sometimes, when we compile the Java source code, the compiler may display a warning message  “unchecked conversion” or "  The expression of type List needs unchecked conversion .

In this tutorial, we will study warning messages in more depth. We will discuss the meaning of this warning, the problems it may cause, and how to resolve potential problems.

2.Enable the  Unchecked warning option

Before studying the "  unchecked conversion " warning, we want to make sure that the Java compiler option for printing this warning is enabled.

If we are using  Eclipse JDT Compiler  , this warning will be enabled by default.

When we use Oracle or OpenJDK javac compiler, we can  -Xlint:unchecked. enable this warning by adding compiler options -Xlint:unchecked.

Usually, we write and build Java programs in the IDE. We can add this option in the IDE's compiler settings.

For example, the following screenshot shows how  to  enable this warning  in  JetBrains IntelliJ  :

Apache Maven is a widely used tool for building Java applications. We can configure  maven-compiler-plugin the  compilerArguments enable this option:

Now that we have confirmed that the Java compiler has enabled this warning option, let us take a closer look at the warning.

3.When will the compiler warn us: "  unchecked conversion "?

In the previous section, we learned how to enable warnings by setting Java compiler options. Therefore, it is not difficult to imagine **  “unchecked conversion” is a compile-time warning.  Typically,  in the case of the original type assigned to a parameterized type without type checking  ,  we'll see this warning.  **

The compiler  allows this assignment  because the  compiler must allow this assignment to maintain backward compatibility with older Java versions that do not support generics  .

An example will quickly explain it. Suppose we have a simple method to return the original type  List :

Next, let's create a test method that calls the method and assigns the result to  List<String> a variable of type :

Now, if we compile the above test, we will see a warning from the Java compiler.

Let's use Maven to build and test our program:

As shown in the output above, we have copied the compiler warning.

A typical example in the real world is the way we use the Java Persistence API  [Query.getResultList()](https://docs.oracle.com/javaee/7/api/javax/persistence/Query.html#getResultList--) . This method returns an  List object of primitive type .

However, when we try to assign a list of primitive types to a list with parameterized types, we see the following warning at compile time:

In addition, we know that if the compiler warns us, it means that there is a potential risk. If we look at the Maven output above, we will see that  unchecked conversion our test method is working , although we receive the " " warning.

Naturally, we may want to ask why the compiler warns us with this message, and what potential problems we might encounter?

Next, let us figure it out.

4.Why does the Java compiler warn us?

Even if we receive the "  unchecked conversion " warning, our test method can work well in the previous section. This is because the  getRawList() method will only be  String added to the return list.

Now, let's change the method a little bit:

In the new  getRawListWithMixedTypes() method, we add the  Date object to the returned list. Since what we return is a list of primitive types that can contain any type, it is allowed.

Next, let's create a new test method to call the  getRawListWithMixedTypes() method and test the return value:

If we run the above test method, we will see the "  unchecked conversion " warning again and the test will pass.

This means that when we call by  get(3) obtaining  Date objects and convert them to type  String. when thrown ClassCastException   String.

In the real world, depending on requirements, sometimes it is too late to throw an exception.

For example, we allocate  List<String> strList = getRawListWithMixedTypes(). for  strList, each  String object  strList, assuming that we use it in a fairly complex or expensive process (such as external API calls or transactional database operations).

When we  strList encounter on an element in  ClassCastException , some elements have already been processed. Therefore,  ClassCastException too late may lead to some additional restoration or data removal process.

So far, we have understood the  “unchecked conversion” potential risks behind the warning. Next, let us see how to avoid this risk.

5.How should we deal with warnings?

If it is allowed to change the method that returns a collection of primitive types, you should consider converting it to a generic method. In this way, type safety will be ensured.

However, it is very likely that when we encounter the "  unchecked conversion " warning, we are using a method in an external library. Let us see what we can do in this situation.

5.1. Prohibit warning

We can use comments to  SuppressWarnings(“unchecked”) cancel the warning.

However, the  annotation  should  only  be used  when we are sure that the type conversion is safe  ,  because it only suppresses the warning message without any type checking. @SuppressWarnings(“unchecked”)

Let's look at an example:

As mentioned earlier, JPA's  Query.getResultList() methods return the original  List object. According to the query, we determined that the original type list can be converted to  List<Object[]> . Therefore, we can add above the assignment statement  @SuppressWarnings to suppress the "  unchecked conversion " warning.

5.2. Check for type conversion before using collections of primitive types

The warning message "  unchecked conversion " means that we should check the conversion before assigning.

To check the type conversion, we can traverse the collection of primitive types and convert each element to our parameterized type. In this way, if the type of some element is wrong, we can get it before actually using the element  ClassCastException .

We can build a general method for type conversion. According to specific requirements, we can deal with it in different ways  ClassCastException .

First, suppose we will filter out elements of the wrong type:

Let's test the above  castList() method through the unit test method:

When we built and executed the test method, the "  unchecked conversion " warning disappeared and the test passed.

Of course, if necessary, we can change the  castList() method to interrupt the type conversion and throw it immediately after detecting the wrong type  ClassCastException :

As always, let's create a unit test method to test the  castList2() method:

If we run the above test method, it will pass. This means that once  rawList there is an element of the wrong type, the  castList2() method will stop the type conversion and throw ClassCastException.

Six, conclusion

In this article, we learned what the "  unchecked conversion " compiler warning is. In addition, we have discussed the reasons for this warning and how to avoid potential risks.

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How to use checkedList method in java.util.Collections

Best java code snippets using java.util . collections . checkedlist (showing top 20 results out of 432).

Interface List<E>

Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2) , and they typically allow multiple null elements if they allow null elements at all. It is not inconceivable that someone might wish to implement a list that prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but we expect this usage to be rare.

The List interface places additional stipulations, beyond those specified in the Collection interface, on the contracts of the iterator , add , remove , equals , and hashCode methods. Declarations for other inherited methods are also included here for convenience.

The List interface provides four methods for positional (indexed) access to list elements. Lists (like Java arrays) are zero based. Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a list is typically preferable to indexing through it if the caller does not know the implementation.

The List interface provides a special iterator, called a ListIterator , that allows element insertion and replacement, and bidirectional access in addition to the normal operations that the Iterator interface provides. A method is provided to obtain a list iterator that starts at a specified position in the list.

The List interface provides two methods to search for a specified object. From a performance standpoint, these methods should be used with caution. In many implementations they will perform costly linear searches.

The List interface provides two methods to efficiently insert and remove multiple elements at an arbitrary point in the list.

Note: While it is permissible for lists to contain themselves as elements, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a list.

Some list implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the list may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

Unmodifiable Lists

• They are unmodifiable . Elements cannot be added, removed, or replaced. Calling any mutator method on the List will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the List's contents to appear to change.
• They disallow null elements. Attempts to create them with null elements result in NullPointerException .
• They are serializable if all elements are serializable.
• The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.
• The lists and their subList views implement the RandomAccess interface.
• They are value-based . Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
• They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework .

• Arrays.asList(Object[])
• Collections.nCopies(int, Object)
• Collections.EMPTY_LIST
• AbstractList
• AbstractSequentialList

Method Summary

Methods declared in interface java.util. collection, methods declared in interface java.lang. iterable, method details.

The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this method must allocate a new array even if this list is backed by an array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the list is set to null . (This is useful in determining the length of the list only if the caller knows that the list does not contain any null elements.)

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a list known to contain only strings. The following code can be used to dump the list into a newly allocated array of String : String[] y = x.toArray(new String[0]); Note that toArray(new Object[0]) is identical in function to toArray() .

Lists that support this operation may place limitations on what elements may be added to this list. In particular, some lists will refuse to add null elements, and others will impose restrictions on the type of elements that may be added. List classes should clearly specify in their documentation any restrictions on what elements may be added.

containsAll

• contains(Object)
• add(Object)
• remove(Object)

All elements in this list must be mutually comparable using the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the list).

If the specified comparator is null then all elements in this list must implement the Comparable interface and the elements' natural ordering should be used.

This list must be modifiable, but need not be resizable.

The implementation takes equal advantage of ascending and descending order in its input array, and can take advantage of ascending and descending order in different parts of the same input array. It is well-suited to merging two or more sorted arrays: simply concatenate the arrays and sort the resulting array.

The implementation was adapted from Tim Peters's list sort for Python ( TimSort ). It uses techniques from Peter McIlroy's "Optimistic Sorting and Information Theoretic Complexity", in Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474, January 1993.

• Object.hashCode()
• Object.equals(Object)
• equals(Object)

lastIndexOf

Listiterator.

This method eliminates the need for explicit range operations (of the sort that commonly exist for arrays). Any operation that expects a list can be used as a range operation by passing a subList view instead of a whole list. For example, the following idiom removes a range of elements from a list: list.subList(from, to).clear(); Similar idioms may be constructed for indexOf and lastIndexOf , and all of the algorithms in the Collections class can be applied to a subList.

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

spliterator

The Spliterator reports Spliterator.SIZED and Spliterator.ORDERED . Implementations should document the reporting of additional characteristic values.

• If the list is an instance of RandomAccess then the default implementation creates a spliterator that traverses elements by invoking the method get(int) . If such invocation results or would result in an IndexOutOfBoundsException then the spliterator will fail-fast and throw a ConcurrentModificationException . If the list is also an instance of AbstractList then the spliterator will use the list's modCount field to provide additional fail-fast behavior.
• Otherwise, the default implementation creates a spliterator from the list's Iterator . The spliterator inherits the fail-fast of the list's iterator.

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